1. Field of the Invention
This invention relates to new photoresist compositions and methods that can provide significantly improved lithographic performance upon coating and imaging on metal substrates.
2. Background Art
Photoresists are photosensitive films for transfer of images to a substrate. They form negative or positive images. After coating a photoresist on a substrate, the coating is exposed through a patterned photomask to a source of activating energy such as ultraviolet light to form a latent image in the photoresist coating. The photomask has areas opaque and transparent to activating radiation that define a desired image to be transferred to the underlying substrate. A relief image is provided by development of the latent image pattern in the resist coating. The use of photoresists is generally described, for example, by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York (1975), and by Moreau, Semiconductor Lithography, Principals, Practices and Materials, Plenum Press, New York (1988).
An important property of a photoresist is image resolution. A developed photoresist image of fine line definition, including lines of sub-micron and sub-half micron dimensions and having vertical or essentially vertical sidewalls is highly desirable to permit accurate transfer of circuit patterns to an underlying substrate. However, many current photoresists are not capable of providing such highly resolved fine line images.
More recently, certain "chemically amplified" photoresist compositions have been reported. Such photoresists may be negative-acting or positive-acting and rely on multiple crosslinking events (in the case of a negative-acting resist) or deprotection reactions (in the case of a positive-acting resist) per unit of photogenerated acid. In other words, the photogenerated acid acts catalytically. In the case of the positive chemically amplified resists, certain cationic photoinitiators have been used to induce cleavage of certain "blocking" groups pendant from a photoresist binder, or cleavage of certain groups that comprise a photoresist binder backbone. See, for example, U.S. Pat. Nos. 5,075,199; 4,968,851; 4,883,740; 4,810,613; and 4,491,628, and Canadian Patent Application 2,001,384. Upon selective cleavage of the blocking group through exposure of a coating layer of such a resist, a polar functional group is provided, e.g., carboxyl, phenol or imide, which results in different solubility characteristics in exposed and unexposed areas of the resist coating layer.
With the desire to produce high-density semiconductor devices, there is a movement in the industry to shorten the wavelength of exposure sources and use deep U.V. radiation. Such photoresists offer the potential of forming images of smaller features than may be possible at longer wavelength exposure. As is recognized by those in the art, "deep UV radiation" refers to exposure radiation having a wavelength in the range of 350 nm or less, more typically in the range of 300 nm or less such as radiation provided by a KrF excimer laser light (248 nm) or an ArF excimer laser light (193 nm).
However, a significant disadvantage with chemically-amplified resists is their frequent sensitivity to the environment as well as the underlying substrate, which can result in reduced resolution of the resist relief image. In particular, resolution problems often occur upon coating onto metal substrates such as TiN. For example, developed images of many current resists applied on TiN or other metal substrates often will exhibit "footing", i.e. where the resist fails to clear during development resulting in an upwardly tapering relief image sidewall. Deposition and processing of resists on metal substrates is required for many microelectronic device fabrications.
It thus would be desirable to have new photoresist compositions and methods that could provide highly resolved fine line images. It would be further desirable to have such new photoresist compositions and methods that provide highly resolved resist relief images upon coating and imaging on metal substrates such as TiN.